Page images
PDF
EPUB

1. Red ferruginous earth and gravel. 2. Streaked yellowish and red clay. * 3. Quartz gravels of moderate size.

4. Large quartz pebbles and boulders ; masses of iron-stone set in very compact clay, hard to work.

5. Blue and white clay. 6. Pipe-clay.

It will be observed that here, as in New South Wales, the gold is accompanied by ferruginous earths. The Romans—who distinguished this formation by the name of Segullum, a word which seems to have some connexion with the Gothic gull, gold-considered its presence as a certain indication of the proximity of gold. The gold of river-sands they called Chrysammos. Although such is the general order of succession in the strata of auriferous alluvia at Ballarat, nothing, Governor Latrobe informs us, is more striking than the irregularity of the proportions in which they are found to be distributed, the variety of inclination obseryable within a limited space, or the unequal depth at which any given stratum may be found to lie below the surface. In some workings the pipe-clay may be reached at the depth of ten or twelve feet, in others, not at thirty or upwards ; circumstances which would be readily appreciated by a man of science, as originating in the peculiar disposition for local alluvial accumulations. Gold, Governor Latrobe further tells us, has been detected, he believes, in all the superior formations, even in the superficial soil. But by far the richest deposit is found in the small veins of blue clay, which lie almost (immediately) above the so-called “ pipeclay,” in which no trace of the ore has been discovered.

We have remarked before that gold has not as yet been traced in sitû in Australia, or if so, to a very insignificant extent; but even if this had been the case, we are decidedly of opinion it is to gold-digging and washing alone that the means and energies, whether of individuals or of companies, should be at present directed. It is, as Sir Roderick I. Murchison has justly remarked, in the abrasion of rocks and extraordinary superficial distribution of large quantities of gold, that lies the providential arrangement by which all the labours and expense of extraction and separation are saved, and the enterprise and emigration of large bodies of men are brought into operation.

The study of the nature and succession of gold-bearing alluvial deposits is as yet in its infancy. Notwithstanding that from the most ancient times gold has been more generally obtained superficially than by rockmining-indeed, it is a geological constant” that mines go on diminishing in their yield-still little has been done towards reducing golddigging to a science. The ancients distinguished, as we have seen, their chrysammos from their segullum. They also knew that sometimes auriferous sands were met with on the surface of the soil, but Pliny describes this as rara felicitas. The gold-washers in Asiatic Russia, in South America, on the Gold Coast of Africa, and in various parts of Europe, have reduced this art to a few well-defined principles, founded on experience obtained in the different localities. Sir Roderick Impey Murchison has described the system pursued by the Russians; Bosman and Mungo Park have given good accounts of the operations carried on on the Gold Coast of Africa; Ulloa, Betagh, the Jesuit Ovalle, Bouguer, and Humboldt, have described the superficial diggings of Central and

66 wet"

of South America. Landrin, in his treatise “De l'Or," has described most of the European lavatories, and has ably condensed the experience obtained by the washers of the Rhone, the Rhine, and other gold-bearing streams in Europe.

In the “Lectures on Gold” delivered at the Museum of Practical Geology, for the instruction of emigrants about to proceed to Australia, Mr. J. Beete Jukes, to whom was allotted the subject of “The Geology of Australia,” gives some practical advice on river alluvia, and on and “ dry” diggings; but their brevity is excessive, barely extending to more than a few lines. Professor Forbes having had “ Rocks” for his subject, he has omitted consideration of alluvia altogether. The other lecturers had “ Chemical Properties," “ Dressing,” “Metallurgical Treatment,” and “History and Statistics,” all of greater or less use to the emigrant. The lecture on chemical properties might perhaps have had a more practical character imparted to it. The fact is, that there are only iron and copper pyrites which can be mistaken by any intelligent person for gold. But both of the former can be cut with a penknife; not so gold. Iron pyrites strikes fire against steel; not so gold. Pyrites dissolve in strong mineral acids; not so gold. Pyrites burn, and give off an odour of sulphur; not so gold. Gold, on its side, amalgamates with quicksilver; not so pyrites. Gold, indeed, should always be known by its colour and lustre.

The lecture on dressing, or mechanical preparation of gold ores, by Mr. W. W. Smyth, is the one which will be found to be of greatest practical value to the emigrant; and we would venture, after carefully perusing that lecture, to offer the following additional advice:

Whether gold occurs in rocks, or sands, or soil, the readiest means of detecting it is by the use of quicksilver. The operation in both cases is the same, only that in the case of gold being intimately disseminated in quartz or other rock (and it is also to be observed that iron pyrites very often contains gold, and must in that case be treated the same as rock), the auriferous rock must, in the first place, be calcined till a magnet passed through the impalpable powder is covered with particles of magnetic iron. The residue is then in the same category as would be auriferous alluvium, clay, gravel, soil, or sands, that have been deprived of moisture by heat, and treated in a similar manner with the magnet. A quantity of quicksilver is then put into a vase, and a portion of the calcined powder, the weight of which has been carefully determined beforehand, is added to it; it is then stirred and washed with distilled water. The stirring may be performed with the magnetic bar, which covers itself with iron particles ; these are successively removed, till no more iron remains to attach itself to the magnet. The residue is then filtered through wash-leather. The water, and the gold which is in a state of amalgamation with the quicksilver, pass through, and are thus separated from the earthy substances.

The quicksilver and water are then placed in a porcelain vase or crucible in a sand bath, which is heated slowly. The water evaporates first, and leaves the quicksilver dry; next the quicksilver passes off in the state of vapour. When, after a prolonged calcining, it is seen that no more quicksilver remains, the weight of the golden residue left in the crucible is determined and compared with that of sand, and the following estimate is made :-If such a weight of rock, sand, or soil, has given such a weight of gold, how much would 100, 1000, or 10,000 of rock, sand, or soil give? It is to be observed, that the assay must be made with scales of great sensitiveness.

All other kinds of assays which are effected with a view to determine the quantity of gold among sandy or earthy substances, are founded upon its great specific gravity, which is only surpassed by platinum, and equalled by tungsten. If, then, we are in search of gold either in sands or alluvial soil, they must, in the first place, be reduced into a state of powder, só as to separate all the mineral substances which are associated with it. In that state, the question reduces itself to seeking for gold in minute particles in a more or less fine sand.

This understood, take a shallow black plate, put in it a small quantity of the sand supposed to contain gold, and pour water upon it till the sand is covered. Then impart to it a gentle movement, so that the water shall be slightly disturbed and carries a few grains of sand to the surface. Take advantage of this moment to pour off the superficial water, and replace it by new. Go through this process with much patience, never throwing too much energy into the operation, till there remains nothing at the bottom of the plate but a clean-washed sand, in which the particles of gold will be readily discerned. The proportion of gold to alluvium or sand can then be readily determined as before, by ascertaining the weight of gold-residue as compared with that of the sand or soil used. But it is to be observed, that in carrying out this result, by estimating if a pailful of sand taken from some productive spot, weighing one pound, produced one drachm of gold, there would be many sources of fallacy in propounding that so many tons or acres of soil would produce so many pounds of precious metal. The comparative produce of the European and Asiatic gold mines is now pretty well known.

Now, in reference to the yet little-studied question of the nature and character of gold-bearing alluvia, it may be observed, that all matters of the same specific gravity stop and form a deposit at a given place : platinum, gold, tungsten, palladium, rhodium, silver, the oxides and titaniates of iron, are first to fall to the bottom, and give rise there to reddish, brownish, or blackish deposits, according to the abundance of one or other of these metals, and which the practised eye knows how to determine at

* Quartz, mica, felspar, &c., derived from the disintegration of the veinstone or the neighbouring rock, and which have nearly the same specific gravity, are stopped in their movement, and precipitated with the heavier metals, but more frequently they are carried to greater distances, and give rise to deposits of a colour that is very readily recognised.

The largest grains or nuggets of gold deposit themselves generally as soon as the course of the waters has ceased to possess the character of a torrent; but the finer particles are carried to a distance; hence it has been remarked, that the more rapid is a stream, the more numerous and the smaller are the grains of gold.

In some localities, as especially on the western slope of the Sierra Nevada, in New California, and as observed by Mr. Stutchbury in one case in Australia, the auriferous rock has been disintegrated by the action of rains, sun, and atmosphere ; the quartz has disaggregated, and the

once.

· Hence it is that the Segullum indicates the presence of gold.

grains of gold are found in their native locality, under all kinds of forms --little veins, crystals, laminæ, &c. The fragments of the veinstone are sometimes sufficiently large to require crushing preliminarily to the washing and subsequent amalgamation.

The grains of gold deposited in sands are of all forms and sizes ; they seldom present themselves in the shape of crystals, and even then the angles are rounded off by the friction to which they have been exposed in their transport, which has sometimes lasted for ages. Gold most frequently occurs in smooth, rounded grains, or flattened into spangles, sometimes of excessive minuteness.

When quartzose veinstones, which contain gold, are crushed and sifted, it is remarked that the grains of metal are the first to break by a natural cleavage which appertains more or less to all metallic substances. This tendency to reduce itself into smaller fragments explains the great tenuity of gold spangles and grains that have been carried a long distance, and incessantly triturated by currents of water, and explains the remarks made by all gold-diggers, that the greater the distance of the auriferous sands from their native site, the smaller are the grains and particles of gold; in the same manner that the more rapid the torrent, the greater is the degree of trituration, and the readier the reduction of the gold into microscopic particles.

According to Agricola (“De Re Metallicâ,” lib. iii., p. 54), ancient mineralogists, among whom he quotes Calbus, believed that a river, to be auriferous, must flow from east to west or from the th to the south; that the riches of rivers increased or diminished according as their course followed these directions more or less directly, and that those which contained the least gold had their sources in the south, and received tributaries from the west. It is useless to refute so ridiculous an idea, as there is no reason that it should be so; even if all the waters which bear along with them auriferous deposits were so circumstanced, still that would not authorise any general deduction. But it is by no means so ; the Rhine and the Ariège flow from the south to the north; the Rhone and the Herault from the north to the south; the Gardon, the Cèse, the Arve, from west to east ; and yet all these rivers are auriferous to an amount that differs

very

little one with the other. It would be much more correct to say, that every time a current of water flows from mountains belonging to the crystalline and older sedimentary formations, and that its sands are chiefly composed of grains of quartz with titaniferous iron, there may be modes of finding particles of gold. But even this aphorism is far from being exact, since the first condition of the existence of gold in the sand of rivers is, that their sources, or those of their tributaries, should encounter veins of auriferous quartz. The Rhone, for a distance of twenty-two leagues, that is to say, from its sources to beyond Lake Geneva, presents no traces of this precious metal; but it becomes auriferous from the point where it receives the little river of Arve ; this originates from the circumstance that the great river does not come in contact with any vein of gold, whilst the Arve flows over such. In the same way the Macquarie is auriferous only after receiving the Turon and Ophir rivers.

About the year 1835, some adventurers who were washing the sands of a rivulet three or four leagues from the Indian village of Navogame, in Mexico, remarked that on ascending the river beyond a certain point, the washings were entirely unproductive, and that the sands afforded no more particles of metal ; they concluded from this that this point must be near to the vein of gold. Some researches were accordingly made on the two acclivities, which were clothed with pine-trees, and a vein of quartz was discovered which contained grains of gold perfectly visible to the naked eye. From that moment a mine was worked at open day, which has since obtained so great a celebrity under the name of Descubridora.

Réaumur, one of the most ingenious observers of nature, remarked, now some time back, that the more rapid is the course of a river, the less easy is it for the particles to be deposited: the water carrying them along till they are sufficiently buried in sand to resist its power. Upon this point depends the different degree of metallic produce of sands. It is where the flow of rivers is least rapid, and at points where their waters spread out, that the greatest abundance of gold may be expected. Every obstacle which breaks the

rapidity of the current is favourable to the deposition of gold : blocks of rock and stones form excellent ramparts for staying their progress; the curves formed by rivers, and which produce back-waters, are the places where the most is precipitated. These are practical points which the gold-washers of the Rhone know well how to take advantage of.

Yet, notwithstanding all this, it is very difficult to say which will be the most profitable portion of a river to work ; in the Altai, it often happens that of two washings, situated the one above the other on the same river, that which is the most distant from the source contains the most auriferous sands.

This may arise from various circumstances; from, as has been before explained, the force of the current ; from the action of floods, or from the gold-sand being derived either from slopes of hills or rivers or rivulets joining the main stream from transverse valleys, or rivulets flowing between two spurs of a mountain-range ; or from longitudinal valleys or rivulets flowing between the central chain and an outlying range, or between two outlying ranges, according as the main stream itself flows along a transverse or a longitudinal valley. A river flowing through an open country, not itself auriferous, may receive tributaries rich in golddust from both longitudinal and transverse valleys. Generally speaking, rivulets flowing through transverse valleys should present greater likelihood of auriferous sands, because they cut through the metamorphic rocks at right angles to their dip, thus exposing a greater amount and variety of formations ; whereas rivers and rivulets flowing along a longitudinal valley, often follow the same line between two similar beds or between two successively tilted-up formations for a great distance. The most favourable point to explore the latter is where they themselves become transverse rivulets, and leave one longitudinal valley to pass into another or into the open country: Instances occur, to judge by Arrowsmith's map of the auriferous districts of New South Wales, of two rivulets flowing along transverse valleys nearly parallel to one another, and each bearing auriferous sands. In such a case, it is evident that all the probabilities of the case are that the native site of the gold is in the spur that separates the two rivulets from one another; for all the chances are against two rivulets, flowing along different but parallel valleys, deriving

« PreviousContinue »